getClosedIssues uses since to grab only the needed issues:

const { data: issues } = await this.issues.listIssues({
  state: 'closed',
  since: releaseRanges[releaseRanges.length - 1][1].date
});

getMergedPullRequests should do the same instead of paginating over all pull requests in the repo, which can take a while for large projects. If there's no since param, maybe there's a way to sort by merge time, or we could could potentially use listIssues as an initial list of all the PR issues that have been updated since the date, and then short-circuit out of getMergedPullRequests once it's seen all of them? 🤷‍